Working Principle of Liquid (Water) Ring Vacuum Pumps

The liquid-ring vacuum pump (often referred to as the water-ring pump) is a rough vacuum pump capable of achieving a ultimate vacuum of 2,000 to 4,000 Pa; when used in series with an atmospheric ejector, it can attain a vacuum as low as 270 to 670 Pa. The water-ring pump can also serve as a compressor—known as a liquid-ring compressor—which is a low-pressure compressor with a pressure range of 1 to 2 × 10⁵ Pa gauge pressure.

Initially used as a self-priming water pump, the liquid-ring pump has gradually found applications in numerous industrial sectors, including petroleum, chemical engineering, machinery, mining, light industry, pharmaceuticals, and food processing. In many industrial production processes—such as vacuum filtration, vacuum water lifting, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum rehumidification, and vacuum degassing—the liquid-ring pump is widely employed. Thanks to the rapid advancement of vacuum technology, the liquid-ring pump has long been highly regarded for its ability to achieve rough vacuum levels. Since gas compression in a liquid-ring pump occurs under isothermal conditions, it can efficiently handle flammable and explosive gases. Moreover, it can also remove gases containing dust and moisture. As a result, the application of liquid-ring pumps continues to grow.

The working fluid used in liquid-ring vacuum pumps can also be other suitable liquids; regardless of the specific working fluid employed, these pumps are collectively referred to as liquid-ring vacuum pumps. The water-ring vacuum pump is the most common and widely used type of liquid-ring vacuum pump.

As shown in the figure, an appropriate amount of water is filled into the pump body as the working fluid. When the impeller rotates clockwise as indicated in the figure, the water is thrown outward by the impeller. Under the action of centrifugal force, the water forms a nearly uniform-thickness closed ring whose shape is determined by the geometry of the pump cavity. The inner surface of the lower part of the water ring is precisely tangent to the hub of the impeller, while the inner surface of the upper part of the water ring just touches the tips of the blades (in reality, the blades are inserted to a certain depth into the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the inner interface of the water ring; this space is further divided by the impeller blades into several small chambers, each corresponding to the number of blades. If we take the lower part of the impeller at 0° as the reference point, then as the impeller rotates through the first 180°, the volume of each small chamber gradually increases and becomes connected to the intake port on the end face. At this stage, gas is drawn into the chamber. When the intake process is complete, the small chamber is sealed off from the intake port. As the impeller continues to rotate, the volume of the small chamber gradually decreases, thereby compressing the gas inside. When the small chamber aligns with the exhaust port, the gas is expelled from the pump.

In summary, the liquid-ring pump achieves intake, compression, and exhaust by varying the volume of the pump chamber; therefore, it is classified as a variable-displacement vacuum pump.

Compared with other types of mechanical vacuum pumps, liquid-ring pumps have the following advantages:

Simple structure, low manufacturing precision requirements, and easy to machine.

It has a compact structure and a relatively high pump speed; generally, it can be directly connected to an electric motor without the need for a reduction gear.

Therefore, by using small structural dimensions, it is possible to achieve a large displacement while occupying a small footprint.

Compressed gases are essentially isothermal, meaning that the temperature change during the compression process is minimal.

Since there are no metal friction surfaces inside the pump cavity, the pump does not require internal lubrication and experiences very little wear. The seal between the rotating and stationary components can be achieved directly by a water seal.

Even inhalation, smooth and reliable operation, simple operation, and easy maintenance.

Water-ring pumps also have their drawbacks:

Low efficiency, typically around 30%, with better ones reaching up to 50%.

The low vacuum level is not only due to structural limitations but, more importantly, is constrained by the saturation vapor pressure of the working fluid. When water is used as the working fluid, the ultimate vacuum pressure can reach only 2,000 to 4,000 Pa. When oil is used as the working fluid, the ultimate vacuum pressure can attain as low as 130 Pa.

In summary, because gas compression in a liquid-ring pump is isothermal, it can handle flammable and explosive gases. Moreover, since the pump has no exhaust valve and no friction surfaces, it can also extract gases containing dust, condensable gases, and gas-water mixtures. Despite its relatively low efficiency, this pump has found widespread application thanks to these outstanding features.

Cavitation in Liquid Ring Vacuum Pumps

When a liquid-ring vacuum pump is operating, at a certain temperature, if the liquid pressure in the low-pressure region falls below the vaporization pressure (i.e., the saturation vapor pressure) at that temperature, the liquid begins to vaporize and form bubbles. As these bubbles move with the liquid into the high-pressure region, they collapse, and the surrounding liquid rapidly fills the void left by the collapsing bubbles, generating a hydraulic shock. This phenomenon—of bubble formation, growth, and collapse—is known as cavitation.

During the operation of a liquid-ring vacuum pump, the absolute pressure in the suction zone is identical to the absolute pressure of the system being evacuated. In other words, the higher the vacuum level of the system being evacuated, the higher the vacuum level in the suction zone as well.

Liquid-ring vacuum pumps require a liquid as the working medium. Each liquid has a corresponding saturation vapor pressure at a given temperature. The closer the absolute pressure in the suction region is to the liquid's saturation vapor pressure, the closer the liquid gets to a boiling state. At this point, numerous vapor bubbles will form on the surface of the working fluid in the suction region. Since the vapor generated by the working fluid within the pump chamber occupies part of the chamber's volume, the pump's external suction capacity will be reduced. When the pressure in the suction region reaches the saturation vapor pressure of the working fluid, it can be considered that the entire pump chamber is filled with working-fluid vapor, at which point the pump's external suction capacity approaches zero. It is also at this moment that cavitation in the pump becomes most severe.

If the working fluid is water, the higher the water temperature, the greater the decline in the pump's pumping performance. The effect of water temperature on the pumping rate of a liquid-ring vacuum pump can be referred to page 72 of this manual.

The cavitation damage in liquid-ring pumps and that in centrifugal pumps follows the same principle: at the locations where vapor bubbles form and collapse, pitting corrosion appears on the metal surface, and in severe cases, honeycomb-like damage may develop. If the impeller of a vacuum pump has significant residual stress at the cavitation sites, this can lead to stress relief and the formation of cracks.

When cavitation occurs in a pump, bubbles continuously and suddenly collapse in high-pressure regions, accompanied by intense water hammer, which generates noise and vibration. The sound produced can be heard as a crackling or popping noise, reminiscent of popping beans. Measured results indicate that the frequency range of vibrations caused by cavitation spans from 600 to 25,000 Hz, with pressures reaching up to 49 MPa.

If the aforementioned bubbles rupture on a metal surface, the metal surface will be subjected to continuous and intense water hammering, resulting in pitting. The metal grains will become loosened and flake off, forming a honeycomb-like structure, and in severe cases, even leading to perforation. In addition to mechanical effects, cavitation damage is accompanied by a variety of complex phenomena, including electrolysis and chemical corrosion. Actual damage patterns show that the areas of cavitation damage on pump flow components are precisely those where the bubbles disappear.

Preventive Measures and Elimination Methods for Cavitation Phenomenon

When a liquid-ring vacuum pump operates for an extended period under cavitation conditions, it can lead to increased noise and vibration, and may even cause damage to the pump. Therefore, it is advisable to avoid operating the liquid-ring vacuum pump under cavitation conditions as much as possible. This can be achieved through the following methods:

1. Lower the temperature of the working fluid.

Under the premise that the operating point of the liquid-ring vacuum pump is fixed, lowering the temperature of the working fluid can effectively prevent cavitation.

2. Operate the vacuum pump within its safe operating range to improve the accuracy of selection.

This requires that, during the selection phase, when choosing the suction pressure and working fluid temperature for liquid-ring vacuum pumps, one should as much as possible avoid pressure ranges where liquid-ring pumps are prone to cavitation.

3. Install cavitation protection piping or a check valve.

When the bubbles generated during cavitation collapse during compression, introducing a non-condensable gas at higher pressure from the outside can promptly replenish the space created by the bubble collapse. This significantly reduces the damage caused by cavitation to the pump and also lowers the noise and vibration induced by cavitation.

The 2BV and 2BE1 models are equipped with cavitation protection interfaces. When cavitation occurs in the pump, simply introducing a non-condensable gas through the cavitation protection interface on the pump’s exhaust side can eliminate cavitation noise and significantly reduce the damage caused by cavitation to the pump.

4. Equipped with an atmospheric ejector

If the suction port of a liquid-ring vacuum pump is equipped with an atmospheric ejector, the liquid-ring pump is less likely to experience cavitation.

5. Use a liquid with a lower saturation vapor pressure as the working fluid.

6. The pump’s impeller and disks are made of materials with strong resistance to cavitation.